textproduct: Rapid City

KEY MESSAGES

- Strong to severe thunderstorms are expected Saturday afternoon and evening with primary hazards of large hail quarter to golf ball size, damaging winds in excess of 60 MPH, and heavy rainfall.

- Unsettled conditions with daily chances of showers and thunderstorms continue into the middle of next week.

UPDATE

Issued at 1053 PM MDT Fri May 29 2026

Modest SBCAPE values of 1000-1500 J/kg have fueled storms across northeastern WY this afternoon and evening. While a few of the stronger cells have produced some small hail and gusty winds, these storms remain sub-severe and are "pulsey" in nature as bulk shear is almost non-existent. Chances for scattered showers/storms will continue over northeastern WY and into western SD as weak lead shortwave moves across the region overnight and moisture advection into the CWA ramps up. Not anticipating any severe weather overnight, though a few of the stronger cells could produce some small hail and gusty winds.

Forecast remains on track for storms and the potential severe weather tomorrow. Moderate instability (CAPE of 2000-3000 J/kg) will develop in the afternoon with southeast flow bringing in plenty of moisture. An approaching shortwave will provide enough forcing to get storms going. Deep layer shear of 20-30 kts will limit supercell potential but will support organized multicells with large hail and damaging wind as the primary threats, though a weak tornado or two can't be ruled out. High PWATS (exceeding 1.00") will support heavy rain potential in these storms.

DISCUSSION

(This Evening Through Friday) Issued at 254 PM MDT Fri May 29 2026

Hard to rule out an isolated thunderstorm firing this afternoon/evening over the Black Hills region or across Campbell County Wyoming, but otherwise one more warm and breezy day is in store for northeastern Wyoming and western South Dakota. Some scattered elevated shower activity with embedded thunder is also possible tonight into Saturday morning, especially across our South Dakota plains.

The main show remains Saturday, with SPC maintaining a Slight Risk of severe thunderstorms across our central and southern South Dakota plains, and a Marginal Risk for just about the rest of the CWA. Signals are "mixed" with regard to the severe weather potential...not a slam-dunk or high-end setup by any means, yet still potential hazards to discuss. Lift won't be an issue...storms are expected to fire off the northern arc of a dryline into the Nebraska panhandle, and ahead of an approaching negatively-tilted trough lifting across Wyoming. Moisture also won't be a problem...surface moisture advection is underway today with dew points already exceeding 50 degF in our southeast counties, and a strong south-southeast flow throughout our atmospheric column will continue to stream moisture northward tonight and Saturday. Surface and near-surface winds backed to southeasterly Saturday will assist in transporting this moisture westward and pooling it up against the Black Hills corridor, as well as enhance storm-relative inflow. Tds will remain lower west of the Black Hills across northeastern Wyoming, yet still sufficient to support convection with the strongest synoptic forcing approaching from the west. Instability (the proclivity of air parcels to want to rise, aka fuel for storms) won't really be a problem either...RAP and NAMNest forecast sounding support 1,500 to 2,500 J/kg SBCAPE with steep low-level lapse rates, and even the blended HREF mean reaches at least 1,000 to 1,500 J/kg, with the greatest instability focused from the Black Hills eastward across the South Dakota plains given the influx of moisture in this region. However, what will be lacking in this setup is wind shear. No model guidance is broadly reaching 30kts of deep layer shear or EBWD, and 0-1km and 0-3km shear and storm relative helicity values stay fairly low despite sfc winds backed into the southeast across the South Dakota plains. This keeps our region solidly in a multi- cell to cluster regime as storms initiate...local enhancements to directional and/or speed shear will be needed to nudge a cell to transient supercell status. Overall, the lack of shear will do very little to assist in storm organization/longevity or provide good updraft/downdraft separation (yet forcing looks good to sustain convection), suggesting our severe weather risk may be more "pulsy" and episodic in nature throughout the event.

Looking at some more parameters by hazard...brief tornado risk seems low given a lack of a dominant supercell storm mode and weak low- level shear and helicity, but not necessarily zero, especially if any storm mergers or "nudgers" occur or terrain or storm outflows can locally enhance shear. Despite not hitting our ideal target for EBWD and low SHIP values, decent CAPE in the hail growth zone per model forecast soundings and 700-500mb lapse rates of 7.0-8.0 degC/km may allow stronger cells to pulse and produce a few instances of quarter to golf ball size hail, and we continue to see severe hail analogs in this range appearing on the forecast soundings today. Environmental parameters aren't perfect for damaging winds either, but PWATs and DCAPE may be marginally high enough to support some wet microbursts (gusts 50-70 MPH), and the HREF ensemble max wind gust does support this with a few storms. Finally, PWATs exceeding 1.00 inch (especially east of the Black Hills) not only exceed the 90th percentile but approach climatological maximums for late May per SPC sounding climatology, so very heavy rainfall is expected. Overall storms should be moving progressively reducing the flood risk, but initial cells from the Black Hills westward may be slower and may cluster, and we'll also have to watch for any spots that experience multiple storm hits or cell training for a localized risk of flash flooding.

Given all of the above and themes noted in the CAMs, general expectation is for initial storms to fire across northeastern Wyoming and the Black Hills between noon and 2 PM with additional cells inbound from the Nebraska panhandle around this time or shortly thereafter as well, perhaps posing initial primary risks of large hail and very heavy rain, before everything begins to merge and grow upscale into a bow/line of convection as it crosses the Black Hills and tracks northeast across our South Dakota plains over the afternoon and evening with slightly less hail threat but more of a damaging wind threat, perhaps not clearing our eastern counties until midnight or so. This line is also expected to lay down 0.40 to 0.90 inches of rainfall as it tracks northeast, with locally higher amounts.

Another round of showers and thunderstorms is expected Sunday afternoon and evening with a Marginal Risk of severe thunderstorms across portions of our South Dakota plains, but overall coverage and intensity will likely be less compared to Saturday. Enough instability may still be available to support marginally-severe hail or gusty winds if a few robust cells can develop. Unsettled weather still looks to continue well into the middle of next week with daily chances of showers/storms, and temperatures remaining in the 70s and 80s.

AVIATION

(For the 18Z TAFS Through 18Z Sunday) Issued At 1108 AM MDT Sat May 30 2026

Main aviation concern across the KRAP and KGCC terminals will be the development of thunderstorms. Thunderstorm impacts for the KGCC terminal will be between 19-23z and 23-03z across the KRAP terminal. Expect gusty and erratic wind gusts in and around thunderstorms. Lingering -SHRA will continue across the KRAP terminals throughout the evening. Outside of thunderstorm impacts VFR conditions will be held across the terminals through 10z. By 11z there is a chance for some MVFR CIGS to impact the KGCC terminal.

UNR WATCHES/WARNINGS/ADVISORIES

SD...None. WY...None.